Definition
The haversine of an angle is (1 minus cosine) divided by two. The formula states hav(d) equals hav(lat2 minus lat1) plus cos(lat1) cos(lat2) hav(difference of longitude). Because it works with half-angle sines rather than cosines near unity, it avoids the rounding loss that spoils the cosine rule over a few miles. Most navigation software and calculator routines use it, or the equivalent Vincenty method on the ellipsoid for higher precision.
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